The large uptake of mobile broadband has resulted in significantly increased traffic volumes that need to be handled by the networks (e.g. WCDMA/HSPA). Therefore, techniques that allow cellular operators to manage their network more efficiently are of large importance. Some techniques that may help to improve the downlink performance would be for example 4-branch MIMO, multiflow communication, multi carrier deployment etc. Since the spectral efficiency per link is approaching theoretical limits, one next step may be about improving the spectral efficiency per unit area. In other words, additional features for HSDPA need to provide a uniform user experience anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP is working on this aspect by considering heterogeneous network deployments telecommunication systems, cf. for example 3GPP document RP-121436, “Proposed SID: Study on UMTS Heterogeneous Networks” and 3GPP TR 25.800, “Technical report: Study on UMTS heterogeneous networks”, v 2.0.0, August 2013.
Deployment of low-power nodes (LPNs) is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit (RRU), pico, or micro base station, allowing expanding the network capacity in a cost-efficient way. Note that the power transmitted by these LPNs is relatively small compared to that of macro base stations, e.g. 2W as compared 40W for a typical macro base station. A network consisting of traditional macro NodeBs and LPNs is referred to as a heterogeneous network. Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localized traffic hotspots.
Deployed LPNs in a heterogeneous network are typically classified as either co-channel, meaning that each LPN has its own cell identity (scrambling code). LPNs and Macros are different cells but they typically share the same frequency, or combined cell, meaning that the LPNs have the same cell identities as the Macro cell.
Both types of deployment have their respective advantages and disadvantages. In order to optimize performance in a cell, it is of importance to determine which user equipments in a cell are to be served by which LPN, or more general by which radio unit.